Passive Layer Degradation
Passive layer degradation is the loss of the protective oxide layer on passivatable metals due to chemical attack or mechanical stress, which reactivates the base material and increases material removal.
Physico-chemical mechanism of action
Passive layer degradation occurs on passivatable metals such as steel. This layer consists of a dense oxide film that protects the material from corrosion. When this layer is damaged by chemical attack (e.g., hydrochloric acid) or mechanical stress (e.g., friction), the base material can react and start to corrode.
Metallurgical relevance & risks during welding
Degradation of the passive layer increases the material's susceptibility to corrosion. This can lead to cracks, deformations, and reduced service life. In the weld pool, the material can corrode quickly if the oxide layer is not protected, leading to incomplete weld joints or even melting of the metal wall.
Workshop practice & instructions
To avoid passive layer degradation, the following measures should be taken:
• Use clean welding materials and tools.
• Keep the weld pool dry and protect it from chemical attack.
• Apply a passivating treatment (e.g., chromating) to the metal before welding.
• Cool the weld slowly to restore the passive layer.